Method for measuring position and position measuring device for carrying out said method

ABSTRACT

A position measuring system that includes a scale and a scanning device that scans the scale. A light source, which emits a light pulse upon receipt of a request signal and an optical fiber that transmits the light pulse from the light source to the scanning device and for illuminating the scale. At least one photo detector that detects the light pulse affected by the scale as a function of its position.

Applicants claim, under 35 U.S.C. §§120 and 365, the benefit of priorityof the filing date of Oct. 13, 2001 of a Patent Cooperation Treatypatent application, copy attached, Serial Number PCT/EP01/11857, filedon the aforementioned date, the entire contents of which areincorporated herein by reference, wherein Patent Cooperation Treatypatent application Serial Number PCT/EP01/11857 was not published underPCT Article 21(2) in English.

Applicants claim, under 35 U.S.C. §119, the benefit of priority of thefiling date of Oct. 31, 2000 of a German patent application, copyattached, Serial Number 100 54 062.7, filed on the aforementioned date,the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

Incremental or absolute position measuring systems are used fordetecting the definite position of moved objects on machines, such asmachine tools or wafer steppers, for example. In connection with this,the position measuring system must measure the position of the object atfixed definite times and inform the electronic control device whichcontrols the movement sequence. The times are mostly defined by theelectronic control device with the aid of trigger pulses. These triggerpulses are provided to the position measuring system, or to itselectronic evaluation device, which then stores an internal count andtriggers the A/D converters for signal interpolation by storing, ortaking over, instantaneous values of the scanning signals with the sameperiod, which are phase-shifted in respect to each other, and which areanalog-digitally converted. At the end, the internal signal processingdevice in the electronic evaluation device outputs a measured positionvalue which was not present exactly at the time of the triggering, butinstead at a time which was displaced by the amount of storage time.Typical storage times are a few μs.

The continuously increasing displacement speeds, and the increaseddemands made on accuracy at the same time require, particularly at highspeeds, increasingly shorter storing times, and above all an extremelysmall fluctuation of the storing time (storing jitter). The latter willbe explained by the example of a wafer stepper, which was taken from anarticle by P. Kwan, U. Mickan, M. Hercher “NanometergenauePositionsmessung in allen Freiheitsgraden” (Position Measuring AccurateDown to a Nanometer in all Degrees of Freedom), F&M 108 (2000) 9, pp. 60to 64. At a displacement speed of 2 m/s and a storing jitter of only 1ns, the position uncertainty caused by this is already 2 nm, whichrepresents a considerable loss of accuracy in connection with suchapplications. On the other hand, a storing jitter of less than 1 nsmakes extremely high demands on the electronic evaluation device and theposition measuring system. The following effects must be taken intoaccount in connection with position measuring systems:

-   -   i) All analog amplifiers required for signal processing upstream        of the A/D converter have limited bandwidths, and therefore        delay the scanning signals to a considerable extent. Small        amounts of drift of the components used because of the effects        of temperature or aging affect the signal running times and        therefore greatly contribute to storing jitter. Moreover, the        signal running times are a function of the input frequency, and        therefore the displacement speed, which can produce additional        contributions to the storing jitter;    -   ii) The A/D converters also contribute to the storing jitter,        because they do not measure the applied voltages exactly in        relation to the switching flanks of the carrier pulses; and    -   iii) As a rule, the scanning signals are phase-shifted by 90° in        relation to each other. The sine signal, as well as the cosine        signal, must have the same storing time, otherwise an effective        storing time of the position measuring system which differs from        the exact position is obtained, which fluctuates between the        storing time of the sine signal and the cosine signal. Regarding        the position determination within a signal period, the        respective scanning signal located in the vicinity of its        crossover is decisive, since it shows the greatest change in        position, or phase relation, in this range.

A position measuring system is described in DE 44 10 955 A1, in whichthe light source is supplied with a strong current at the time anexternal trigger signal is present as a request signal. The disclosedsynchronization of the light source with external trigger pulses issuited only for low demands made on the storing jitter, because thesupply of the trigger pulses (request signal) to the light sourcelocated in the position measuring system takes place there by a wireconnection from an external electronic tracking device (electroniccontrol device). With customary cable lengths of 0.5 to 20 m, this doesnot assure a sufficient running time stability in the 10 ns range andbelow, and can therefore not be used in demanding applications. In thisconnection it should be noted that low-jitter trigger pulses are mostlyavailable only directly at the electronic components which generatethem.

SUMMARY AND OBJECTS OF THE INVENTION

It is therefore an object of the present invention to disclose a methodfor position measurement and a position measuring system for executingthe method, which assure a precise position measurement.

This object is attained by a method for position determination thatincludes generating a light pulse upon receipt of a request signal andtransmitting the light pulse through an optical fiber to a scale. Themethod further includes illuminating the scale with the light pulse,affecting the light pulse in a position-dependent manner by the scaleand detecting the light pulse affected by the scale by at least onephoto detector.

This object is also attained by a position measuring system thatincludes a scale and a scanning device that scans the scale. A lightsource, which emits a light pulse upon receipt of a request signal andan optical fiber that transmits the light pulse from the light source tothe scanning device and for illuminating the scale. At least one photodetector that detects the light pulse affected by the scale as afunction of its position.

Extremely small storing jitters can be achieved by the measures inaccordance with the present invention.

The present invention will be explained in greater detail by exemplaryembodiments.

Shown are in:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a first embodiment of a position measuringsystem in accordance with the present invention; and

FIG. 2 schematically shows a second embodiment of a position measuringsystem in accordance with the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S) OF THE INVENTION

The position measuring system 100 in FIG. 1 includes a scale 1 with agraduation 2, which can be opto-electrically scanned and moved relativeto a scanning device 5 in the measuring direction X. The graduation 2can be embodied so that it can be scanned by transmitted light or byincident light. The scale 1 can furthermore be designed for incrementalor absolute linear or angular measuring. The light required forilluminating the scale 1 is conducted through an optical fiber 3 to theposition measuring system 100. The light source 4 of the opticalposition measuring system of the invention is not installed directly inthe scanning device 5 and instead is located at the place where alow-jitter trigger pulse I—also called a request signal—is available,i.e. preferably in the vicinity of, or integrated into an electroniccontrol device 200, which generates the trigger pulse I. The triggerpulse I is now transmitted via a digital, and therefore fast driverstage 6 to the light source 4, which therefore synchronously transmits alight pulse IL of a pulse length of 25 psec to 5 nsec. In the electroniccontrol device 200, this light pulse IL is coupled into the opticalfiber 3 and transmitted to the scanning device 5. In this case therunning time of the light pulse IL in the optical fiber 3 is veryconstant, in particular if single-mode fibers are employed. However,very good results are also achieved with multi-mode graded index fibers.In the scanning device 5, the light pulses IL are directed by a suitableoptical scanning device onto the scale 1, and finally to photodiodes7.1, 7.2, 7.3. In this case it is irrelevant which optical scanningdevice is used. In particular, imaging, as well as interferentialscanning methods are available. Photo-charges are now generated in thephotodiodes 7.1, 7.2, 7.3 by the short light pulses IL, which aretransmitted to downstream-connected charge amplifiers 8.1, 8.2, 8.3. Thecharge amplifiers 8.1, 8.2, 8.3 can be integrated either into thedownstream-connected electronic evaluation device 9, or directly intothe scanning device 5. It is advantageous if they are integrated intothe scanning device 5, so that charge- amplified scanning signals areconducted through lines 21, 22, 23. At their outlets they provide signalvoltages, which are converted into digital signals by analog-digitalconverters (A/DCs). The subsequent digital processing takes place in thesame way as in customary position measuring systems in that within aperiod interpolation signals are determined from several scanningsignals of the same period, which are phase-shifted by 120° or 90° withrespect to each other. These processing methods are known and will notbe further explained here. Immediately following the measurement, thecharge capacitors of the charge amplifiers 8.1, 8.2, 8.3 are reset(discharged), so that the next measurement can be performed by sending afurther light pulse IL to the scale 1. The short light pulse IL definesin an extremely exact manner the time of the position determination. Thedownstream-located electronic components—such as photodiodes 7.1, 7.2,7.3, amplifiers, A/DCs 10, connecting cables, etc.—do not affect theresult and can therefore be designed to be relatively slow, andtherefore cost-effective.

Depending on the scanning method, laser diodes, VCSELs, LEDs, solidstate lasers, superluminescence diodes, can be considered as lightsources 4. In this case a further advantage of the present inventioncomes to the fore, particularly in connection with semiconductor lasers:because of the pulsing, the laser becomes longitudinally monomodal andcan therefore no longer have interfering mode jumps. In connection withinterferential position measuring systems 100, such mode jumps causesudden and very interfering jumps in the indicated position already withslightly different optical path lengths of the interfering lightbeams.

For incremental scanning methods it is furthermore necessary todetermine the number of the crossed signal periods. In contrast toposition measuring systems in which the scale 1 is continuouslyilluminated, the realization of counters is here no longer directlypossible, since no scanning signals are available between two triggerpulses I. It is therefore suggested to trigger the position measuringsystem 100 very frequently, adapted to signal period and the maximumspeed, or acceleration, for example with 1 MHz. By this high triggerrate it is possible to calculate speeds which can hardly change from onetrigger time to the next from the interpolated fine positions. At a slowspeed a sufficient number of measured values is obtained in one signalperiod so that it is possible to unequivocally detect a jump to the nextsignal period, and a software period counter can therefore count up. Ifthe speed is increased, successive measured values can be separated fromeach other up to a few signal periods, and a dependable count of thesignal periods with the aid of the software period counter is possiblein spite of this. To this end, the speed is approximately calculatedfrom previous fine positions, and the occurring signal periods betweentwo trigger times are determined from this. A high trigger rate isadvantageous. If the trigger rate of the electronic control device 200is too low, the electronic evaluation device 9 must intersperseadditional trigger pulses I.

The exemplary embodiment in accordance with FIG. 2 differs from thefirst one by the use of multi-mode optical fibers 11, 12, 13, for thereturn of the light from the scanning light beams, which were modulatedat the scale 1, to the electronic evaluation device 9. In this case thephotodiodes 7.1, 7.2, 7.3 are contained in the electronic evaluationdevice 9 for receiving the light transmitted back by the optical fibers11, 12, 13. In this way the scanning device 5 becomes passive, i.e. itis no longer connected by electrical cables and can therefore also beemployed in critical environments (high tension, discharges, explosivegases) without interference. Thereby it is furthermore also possible totransmit signals of relatively high frequency in a simple way.

The light pulse IL from the light source 4 can be used for thesimultaneous illumination of several scales, for example on severalshafts of a machine. In this case the optical fibers in which the commonlight pulse IL is supplied to the individual scales should have at leastapproximately the same lengths.

The trigger pulse I is an electrical or optical pulse, by which sensors,and if required actuators, of a machine are simultaneously synchronizedin an advantageous manner. Position measuring systems 100 with severalshafts, distance sensors, acceleration sensors and speed sensors, i.e.sensors which are employed for control, are counted among the sensors.The trigger pulse I is synchronized with the control cycle of thecontrol unit of the machine, for example a numerically controlledmachine tool.

If the trigger pulse I already is an optical pulse, the elementsidentified by the reference numerals 4 and 6 in FIGS. 1 and 2 aresuperfluous, an optical processing unit in the form of an optical fiberamplifier, optical switch or optical mixer can be provided in theirplace.

The highly accurate position measurement in accordance with the presentinvention can be combined with a second position measurement. Forexample, constant light of a wavelength differing from the light pulseIL is transmitted through the optical fiber 3, and the graduation 2and/or another graduation, or coding, is illuminated and scanned by thisconstant light. A rough position is then determined by scanning with theconstant light, and this position is refined by the light pulse IL. Inthis case the rough position can be determined by a hardware counter,and the position determination by the light pulse IL takes place bymeans of a software counter, which provides the instantaneous positionon the basis of interpolation values of the scanning signals, which arephase-shifted with respect to each other.

The present invention may be embodied in other forms than thosespecifically disclosed herein without departing from its spirit oressential characteristics. The described embodiments are to beconsidered in all respects only as illustrative and not restrictive, andthe scope of the invention is commensurate with the appended claimsrather than the foregoing description.

1. A position measuring system for position determination of a movableobject of a machine, whose movement is controlled by a regulating unit,the system comprising: a scale; a scanning device that scans said scale;a light source, which emits a light pulse based on a request signal,which is generated by a regulating unit of a machine that has a movableobject, said request signal is generated synchronously with a regulatorcycle of said regulating unit; an optical fiber from said regulatingunit to said scanning device that transmits said light pulse from saidlight source to said scanning device and for illuminating said scale;and at least one photo detector that detects said light pulse affectedby said scale as a function of its position.
 2. The position measuringsystem in accordance with claim 1, further comprising a second scale,wherein a second optical fiber transmits said light pulse so as toilluminate said second scale.
 3. The position measuring system inaccordance with claim 1, further comprising a charge amplifier connecteddownstream of said at least one photo detector.
 4. The positionmeasuring system in accordance with claim 3, wherein said at least onephoto detector is arranged together with said charge amplifier in saidscanning device.
 5. The position measuring system in accordance withclaim 1, wherein said at least one photo detector is arranged in anevaluation device, and said light pulse is affected in aposition-dependent manner by said scale and is conducted via a secondoptical fiber to said at least one photo detector.
 6. The positionmeasuring system in accordance with claim 1, wherein said optical fibercomprises a single-mode fiber.
 7. The position measuring system inaccordance with claim 1, wherein said optical fiber comprises amulti-mode graded index fiber.
 8. The position measuring system inaccordance with claim 1, wherein said light source comprises asemiconductor laser.
 9. The position measuring system in accordance withclaim 1, wherein each of said at least one photo detectors receives apartial light beam of said light pulse affected by said scale.
 10. Amethod for position determination of a movable object of a machine, themethod comprising: generating a request signal by a regulating unit of amachine that controls movement of a movable object of said machine, saidgenerating said request signal is performed synchronously with aregulator cycle of said regulating unit; generating a light pulse basedon said request signal; transmitting said light pulse from saidregulating unit through an optical fiber to a scale; illuminating saidscale with said light pulse; affecting said light pulse in aposition-dependent manner by said scale; and detecting said light pulseaffected by said scale by at least one photo detector.
 11. The method inaccordance with claim 10, wherein said transmitting comprisestransmitting said light pulse from said regulating unit through a secondoptical fiber to a second scale.
 12. The method in accordance with claim10, further comprising generating photo-charges in said at least onephoto detector by said generated light pulses, which are transmitted todownstream-connected charge amplifiers.
 13. The method in accordancewith claim 12, further comprising resetting said charge amplifiersbetween said generated light pulse and a second generated light pulsegenerated after said generated light pulse.
 14. The method in accordancewith claim 10, wherein each of said at least one photo detectorsreceives a partial light beam of said light pulse affected by saidscale.